There have been several models of hermetic electric compressors designed for low-vibration and low-noise application. (As for an example, refer to the patent document 1, Japanese Patent No. 2609713.)
A conventional hermetic electric compressor taught in the above document is described referring to drawings.
FIG. 12 shows the conventional hermetic electric compressor, sectioned vertically, which is referred to in the patent document 1. Referring to FIG. 12, sealed container 1 houses electric compression element 2 and coil spring 3; there is cavity 4 as well in the container. Coil spring 3 is engaged at both ends by snubber 5 protruding from electric compression element 2 side and sealed container 1 side; namely, electric compression element 2 is elastically supported by coil spring 3.
The hermetic electric compressor has been designed to compress the R134a refrigerant, a typical HFC system refrigerant, whose ozone layer destruction factor is zero.
FIG. 13 is noise characteristic chart of the conventional hermetic electric compressor, disclosed in the patent document 1; the lateral axis representing the ⅓ octave frequency, the longitudinal axis the noise level. FIG. 14 details the noise characteristic shown in FIG. 13; where, the lateral axis representing the frequency, the longitudinal axis the noise level.
FIG. 15 shows resonance frequency characteristic of mechanical vibration generated by electric compression element 2 of the conventional hermetic electric compressor; the lateral axis representing the frequency, the longitudinal axis representing level of the acceleration.
The natural resonance frequency due to mechanical vibration generated by electric compression element 2 has been measured by running without load a hermetic electric compressor with the power supply frequency varied, and plotting the acceleration level measured on electric compression element 2, on the frequency axis. The resonance frequency due to mechanical vibration caused by electric compression element 2 is defined as a range of frequencies where the measured acceleration level (vibration level) reach the highest, including the foot areas of the peak in the higher and the lower frequency regions.
FIG. 16 shows resonance frequency characteristic of coil spring 3, in the state where electric compression element 2 is put on coil spring 3; the lateral axis representing the frequency, the longitudinal axis representing the acceleration level. Also shown in the chart is a cavity resonance frequency formed in cavity 4, with R134a used as the refrigerant.
The natural resonance frequency of coil spring 3 has been measured by running without load a hermetic electric compressor with the power supply frequency varied, and plotting the acceleration level measured on the surface of sealed container 1, on the frequency axis. The resonance frequency of coil spring 3 is defined as the range of frequencies where the measured acceleration level (vibration level) reaches the highest, including the foot areas of the peak in the higher and the lower frequency regions.
Now in the following, operation of the above-configured hermetic electric compressor is described.
When power supply is turned ON, electric compression element 2 starts its operation of compressing refrigerant gas. Due to changes of loads and other factors during the compression operation, electric compression element 2 generates mechanical vibrations which contain various frequencies. The mechanical vibration should cause big noises and vibrations if it is conveyed direct to sealed container 1. However, since the elasticity of coil spring 3 absorbs vibration, the vibration which should have been conveyed to sealed container 1 is attenuated. Thus the noises and vibrations are reduced with the hermetic electric compressors.
In the above-described configuration, however, although the mechanical vibrations generated by electric compression element 2 can be absorbed by the elasticity of coil spring 3, the noises and vibrations increase when resonance frequency of the mechanical vibration and that of coil spring 3 coincide, vibration of coil spring 3 is enhanced and resonates at the resonance frequency; the enhanced vibration is propagated to sealed container 1 causing noise and vibration of that frequency. Thus the hermetic electric compressors have had the noise and vibration problem.
Now, a practical example is described. Referring to FIG. 15 and FIG. 16, peak of resonance frequency of the mechanical vibration generated by electric compression element 2 resides at the neighborhood of 540 Hz, which approximately coincides with the peak of resonance frequency of coil spring 3 mounted with electric compression element 2. Since resonance frequency of the mechanical vibration and that of coil spring 3 are in coincidence, the hermetic electric compressor exhibits a noise peak at 540 Hz, as shown in FIG. 14.
On top of the above noise, another noise is generated by the following operation.
Namely, in the conventional hermetic electric compressors, cavity resonance frequency formed in cavity 4 within sealed container 1 resides somewhere at the peak, inclusive of its foot areas, of resonance frequency of coil spring 3 mounted with electric compression element 2.
Referring to FIG. 16, peak of the resonance frequency of coil spring 3 mounted with electric compression element 2 resides at the vicinity of 550 Hz. Also the cavity resonance frequency formed in cavity 4 approximately coincides with the frequency. Furthermore, the hermetic electric compressor has its noise peak in the neighborhood of 550 Hz, as shown in FIG. 14.
The reason for the above is as follows. The mechanical vibration generated by electric compression element 2 vibrates coil spring 3 via upper snubber 5. This creates beating and rubbing between coil spring 3 and the upper and lower snubbers 5. The beating and rubbing is applied on coil spring 3 as vibration energy. Then, coil spring 3 resonates at the inherent resonance frequency of coil spring 3 mounted with electric compression element 2. As the result, noise is generated at the frequency, and the noise vibrates a cavity formed in cavity 4 of sealed container at the resonance frequency. Thus the noise with hermetic electric compressors is enhanced.
Furthermore, if cavity resonance frequency formed in cavity 4 of sealed container 1 coincides with the peak, including the foot areas, of resonance frequency of mechanical vibration generated by electric compression element 2 and resonance frequency of coil spring 3, resonation of coil spring 3 created by the mechanical vibration provides a vibrating effects on cavity 4. Thus the noise due to resonation of the cavity is further increased with the conventional hermetic electric compressors.